Heterodyne receiver



Sept. 5, 1939. o. WOHLFARTH El AL HETERODYNE RECEIVER Filed April 21,1937 INVENTORS Q WOl/ZFARTH AND A TTORNE Y AAlAlAA 'vvvvvv I l I I I l II I l I I I I I I I All I IHIHIHHI Patented Sept. 5, 1939 UNITED STATESHETERODYNE RECEIVER Otto Wohlfarth and Willi Patzschke, Berlin, Germany,assignors to Allgemeine Elektricitatz Gesellschaft, Berlin, GermanyApplication April 21, 1937, Serial No. 138,110 In Germany May 16, 1936 2Claims.

The fundamental operation of heterodyne receiver apparatus known in theprior art in a general way is approximately as follows:

The antenna input circuit consists of a tuned simple or multiple circuitto which the mixer tube is connected. The heterodyne frequency fed tothis tube is generated with an oscillation circuit whose rotarycondenser is actuated jointly with that of the first or input circuit.The beat frequencies resulting from the mixture are further fed tocircuits tuned to a fixed intermediate frequency and are amplified.

After rectification the audio component is fed through additionalamplifier stages to the loudspeaker. Now, inasmuch as the requisitesynchronism between the incoming signal frequency and the oscillatorfrequency is securable only with the expenditure of certain amounts ofmoney and means, and even then only approximately, the suggestion hasbeen made to insure the tuning of heterodyne receiver apparatus only bythe oscillator frequency, the input circuit then being aperiodic. Theoscillator frequency is suitably so chosen that there results acomparatively high intermediate frequency. The selectance of such areceiver is comparatively poor, in short, it does not measure up to whatis attainable with other circuit organizations and with a similar amountof technical and circuit means.

Another drawback of these circuit organizations is that in the input endof the receiver, filters or networks are provided of necessity, in orderthat whistling points, ambiguities, and the reception of short-wavetransmitters may be avoided.

Now, in order to overcome all of these difficulties, it is suggestedaccording to the present invention to leave the oscillator frequencyunaltered and to tune the input circuit; since this results in avariable intermediate frequency, the intermediate frequency portion mustbe designed so as to accommodate in the amplifier a broader band. Theadvantages which results in a given instance is that an outfit thusbuilt has at least the selectivity, with the same number of tubes, as atuned radio frequency receiver, or nonheterodyne set.

The circuit organization distinguishes itself from those heretoforeknown in the art in that no rotary condenser is used for the tuning of acircuit which does not contribute to an improvement of the selectivity.As compared with circuit organizations of the kind before mentionedhaving an aperiodic input, there are in addition obtained the followingmerits:

In order that the apparatus known in theart may be balanced to a givenscale or dial, tuning is required in at least two circuits, namely, thatof the oscillator and that of one or more intermediate circuits. Infact, it is only when both have been balanced and set with the requisiteamount of care the indication will be sufiiciently accurate and thesensitiveness be high enough. In the solution herein disclosed and abovereferred to, adjustment of only one circuit is necessary, and yet atleast the same selectivity and the same accuracy of tuning will beinsured. The receiver could also be furnished with two tuning circuitssynchronized or inter-locked either by that both are designed andconnected as input band-pass filters or that the receiver comprises atuned input stage. Contradistinct to the two-circuit straightaheadreceiver sets known in the earlier art, the double advantage is heresecured, namely, that manipulation is simplified owing to the fact thatfeedback is dispensed with, and that a satisfactorily functioningautomatic volume control is obtainable without any trouble.

Three exemplified embodiments of the heterodyne receiver apparatus heredisclosed are shown in the accompanying drawing.

In the circuit arrangement of Fig. 1, l denotes a combined detector andoscillator or mixing tube, the input circuit 2 being tuned by condenserl0 to the desired carrier frequency, the local oscillation frequencybeing determined by circuit 3 which is preferably tuned to a fixedfrequency for a single wave band. The intermediate frequency currentsappearing in the plate circuit of tube l are fed through the aperiodiccoupling means shown to the signal control grid of tube 4. The amplifiedsignals are impressed across the plate H and cathode l2 of a diodedetector tube 5, the detected component being further amplified in theaudio frequency amplifier tube l3 whose plate circuit I4 is coupled tothe loudspeaker l5.

With a view to improving the selectivity and amplification, the circuitarrangement of Fig. 2 provides a tuned radio frequency stage l6 betweenthe antenna and the resonant signal circuit l of the mixer or firstdetector tube I. The variable condensers which serve to tune gridcircuit 6 and anode circuit 1 to the signal frequency may be operated bya single control means indicated at IT. The aperiodic coupling, which isdesigned to transmit the entire band of intermediate frequency currentspr oduced as circuit 6 is adjusted throughout its tuning range, includesan inductance l8 and a condenser l9 and a resistor 20 connected acrossthe anode and cathode of rectifier tube 5.

The circuit arrangement shown in Fig. 3 is similar to that of Fig. 1except that a tuned band pass filter comprising the resonant circuits 2and 22 areinterposed between theantenna and control grid of tube l, thevariable condensers of which are preferably controlled by a singleoperating means indicated at 23.

While in the examples shown the oscillator frequency remains fixed forreceiving all frequencies within a given wave band, it is within thescope of the present invention to adjust it to a different value foreach wave band while maintaining it constant for all frequencies withina single wave band. It is also within the scope of this invention tochoose the frequency of the local oscillator so high that the variationof the intermediate frequency resulting for reception over a wave bandamounts to only a Smallfraction of the oscillator frequency.

Having described our invention, what We claim as novel and desire tosecure by Letters Patent is: l. A radio receiver comprising thecombination of a tube having a cathode, a signal grid, an oscillatorgrid anda plate, an input circuit tunable over a range of signalfrequencies connected between said signal grid and cathode, means forvarying the potential of said oscillator grid at a fixed frequency, anintermediate frequency amplifier designed to amplify a wide band offrequencies and having a control grid, a cathode and an anode, aresistor connected to said plate and adapted to have one end connectedto a source of positive potential, and a condenser having one sideconnected to said plate and its other side to the control grid of saidamplifier,

sa'id condenser having a low impedance to all currents of the differencefrequencies between the signal frequencies and the fixed oscillatorfrequency.

2. The Vmethod of receiving radio signals covering a band of frequencieswhich comprises the steps of -generating a fixed oscillator frequency,combining the same oscillator frequency successively with thefrequencies of said band, aperiodically amplifying currents of theresulting intermediate frequencies and deriving a direct current voltagefrom the amplified currents and controlling the amplitudeof saidresulting intermediate frequencies with said derived voltage.

WILLI PATZSCHKE. OTTO WOHLFARTH.

